The present invention relates to a speech control apparatus, and more particularly to a speech control apparatus used in a folding telephone.
A conventional folding telephone, which is intended for the enhanced usefulness of portability, includes an upper casing, a lower casing and a hinge. A bottom end of the upper casing is connected to the lower casing by the hinge so that the portable telephone is foldable. When the telephone is used, the upper casing is swung out at the hinge against the lower casing. Otherwise, when the telephone is not used, the upper casing is folded to meet the lower casing so that it is compact and the user can easily carry it.
The folding telephone is designed to have its rotating angle between the upper and lower casings, i.e., the hinge rotating angle, made variable, thus, the speaker in the upper casing and the microphone in the lower casing are located properly at the ear and mouth of the user.
A folding telephone of this kind with the ability of preventing the phenomenon of howling is disclosed in for example, Japanese Laid-open Patent Application No. Heisei 1-212052.
The telephone has a main casing including a speaker, and an arm section including a microphone. The telephone is designed such that when the arm section is swung out by the user to start a communication, the microphone is kept inactive until the arm section has a certain rotating angle with the main casing so that the occurrence of howling which is the leakage of the sound from the speaker into the microphone is prevented.
However, if the user varies the rotating angle between the main casing and arm section during the communication, causing the positional relationship between the user's mouth and the microphone in the arm section to vary, the user's voice level sensed by the microphone will vary. Consequently, the magnitude of the audio signal sent to the telephone of the communication partner will vary, and the partner will hear unfavorably the voice with an abruptly varying level.
Moreover, this telephone is sometimes deficient in the avoidance of a sound leakage from the speaker into the microphone, in which case the partner's voice is unfavorably sent back through the user's microphone as an echo to the partner's telephone.
There is known an echo canceller equipped in a usual telephone. The echo canceller generates an echo replica by estimating the echo path which is the path of sound from the speaker to the microphone on the telephone, and subtracts the echo replica from the speech sensed by the microphone so as to offset the echo.
However, the echo canceller of this type has not been adopted by folding telephone. Even if a folding telephone is equipped with this echo canceller, the echo path cannot be estimated accurately due to the quick variation of the hinge rotating angle, and therefore an accurate echo replica cannot be produced for the cancellation of echo.